Short-scale variations of tectonic regimes in the western European stress province north of the Alps and Pyrenees

Abstract

The data of the European part of the World Stress Map (WSM) are used to study the western European stress province north of the Alps and Pyrenees. The study area is characterised by an almost homogeneous N 145°E orientation of the maximum horizontal compression SH. The tectonic regime experiences short-scale lateral variations from the dominant strike slip (49% of the data) to normal faulting (30%) and to thrust faulting (21%). This is astonishing since the lateral plate boundary forces, responsible for the regional homogeneous stress orientation, are ridge push and Alpine continent-continent collision forces. They create large-scale horizontal compression expressed by thrust and strike slip faulting. The short-scale variations of the tectonic regimes with frequent occurrence of normal faulting led us to consider a tectonic model consisting of upper crustal fragments decoupled from the lithospheric mantle by the ductile lower crust. Lateral plate boundary forces cause these fragments to move independently of each other and of the lithospheric mantle. The lateral boundaries of the fragments fail in thrust, strike slip or normal faulting depending on their orientation with respect to the far-field stress. As a consequence of a differential velocity between brittle upper crust and lithospheric mantle a weak constant shear stress develops in the lower crust. For an estimated upper bound of the differential velocity of 2 mm/year, a low horizontal shear stress of 4.5 MPa is obtained within the lower crust for the study area. Power law rheology together with the temperature increase in the lower crust cause a strong non-linear increase of the horizontal velocity and a concentration of the deformation rate in a 3–5-km-thick transition zone directly above the Moho. A narrow band of strongly reflective heterogeneities within the lower crust in the southern Rhine Graben area coincides with this transition zone. These heterogeneities are therefore regarded as an expression of the deep accommodation zone. The reflective band furthermore forms the lower envelope for deep crustal earthquakes occurring throughout the crust down to ca. 5 km above the Moho in the southern Rhine Graben.